A variety of images such as still images and movies shown on the internet has recently been available owing to the popularization of digital cameras, digital video cameras and the internet. Along with this trend, the demand for easy retrieval of desired images from among such a large amount of images is increasing.
There is known a retrieval system in which images are firstly narrowed down according to tags attached thereto and then only the selected images are analyzed so that the volume of calculation for the image retrieval is reduced (Japanese Patent Laid-open Publication No. 10-091634). The tags are, for example, photography conditions (date/time, location, etc.). It is also known a digital camera which attaches tags to taken images when recording them in media to enhance image retrieval performance. The tags include information on a photographer and environmental conditions at the time of photography (temperature, humidity, illuminance level, etc.) (Japanese Patent Laid-open Publication No. 2006-086823). Moreover, there is known a method for classifying images according to colors and textures of the image as well as information obtained from tags, such as exposure time, f-number, shutter speed, luminance, subject distance, whether a flash was used, etc. (Japanese Patent Laid-open Publication No. 2007-515703).
Even with the above-described techniques, however, the images are not classified or retrieved based on their essential elements, that is, the most notable contents in the image retrieval. The essential elements of the images are, for example, in which event or scene the image was taken or what the main subject of the image is.